Neck Seam/Discoloration Help

[Crese1947]

I'm using a male custom race by ShinglesCat(The Dante one to be specific, although I use them all). It uses vanilla body textures and meshes, so I swapped them out with Skyskins SOS textures, the HIMBO Meshes, and the XMPSSE Skeleton. Thing is, there's a very noticeable neck seam/discoloration between the neck and body of the custom head. I tried to fix it in GIMP, but I couldn't figure out how. Help would be greatly appreciated!

Here's a screenshot: